Iron shielding for telephone cables

Among the benefits obtained by the cabling of telephone circuits is the substantial reduction in magnetically induced voltages that can be realized from the shielding action of the lead sheath. Even greater shielding is attainable if the cable is provided with a steel armor or is installed in iron pipe conduit. In this paper is presented a method for the quantitative prediction of the electromagnetic shielding effected by such iron-surrounded telephone cable sheaths, experimental verification of its applicability, and the necessary impedance data for its utilization.